The X-37B is set to return to Earth after more than a year in space... but its mission remains shrouded in secrecy.
The US Air Force unmanned experimental craft blasted off last March. It measures 8.8 metres, has a wing span of 4.6 metres and looks like a mini space shuttle, and was sent up into space to test its capabilities.
But it is not known what its purpose is, and what it could carry.
That is all classified.
So is it a spy craft monitoring Chinese space station Tiangong 1? Could it carry weapons? Could it transport troops?
"Space-to-space surveillance is a whole new ball game made possible by a finessed group of sensors and sensor suites, which we think the X-37B may be using to maintain a close watch on China's nascent space station," the editor of the British Interplanetary Society's magazine Spaceflight Dr David Baker told the BBC in January.
But Brian Weeden of the Secure World Foundation said that theory was unlikely as the US had other equipment to observe Tiangong 1 and did not need to use the X-37B.
"I would go as far as to say, 'no chance'," Mr Weeden, who was an orbital analyst with the US Air Force, told Space.com. "It's not practical."
He suggested the craft might be testing new technologies that could be used to observe the Middle East and Afghanistan.
One expert even went so far as to say that the plane was "part of a strategic deception program" and had no real purpose other than to puzzle the Chinese government.
"I don't think this thing has a mission that would go beyond bewildering the Chinese," national security analyst John Pike, who founded GlobalSecurity.org, told The Guardian.
"It is the sum of all fears. They don't know what it is and they have to hedge against everything it might be, even though I think it's nothing. It's part of a strategic deception program.
"It looks good; it sounds neat and everything. I assume they put little space experiments, some damn little camera in there. I'm sure the payload bay is chock full of little goodies they're flying on an opportunity basis.
"But, if the thing was actually useful for some national purpose, it would have flown a long time ago because somebody would have been hot to trot."
The X-37B was built by Boeing's Space and Intelligence Systems unit in California. It is the second craft launched into space by the US military.
It is expected to land at the Vandenberg Air Force Base in California in the next few weeks.
In April, the commander of the US Air Force Space Command, General William Shelton, told a symposium that the craft's mission had been a "spectacular success", but would not elaborate on the specifics of the mission, Space.com reported.
The X-37A, its predecessor, made three free-glide flights in 2006, globalsecurity.org reported. It was also developed by Boeing under a US$301 million NASA contract, the site said.
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